A Fish of an Idea

One challenge to generating power from propeller-equipped wind turbines—the giant type used in most wind farms today—is that the devices need their space.

Placed too close together, the turbines suffer from contact with one another’s aerodynamic wakes. To achieve good performance, the sizable turbines must be separated by three or more times their diameters when side by side, says California Institute of Technology biophysicist John Dabiri. A distance of 5 to 10 times their width is needed to the front and to the back. “A mile is not uncommon for downwind spacing of large turbines,” Dabiri says.

This snag limits the amount of power that can be generated per square meter of land on most wind farms. That’s not ideal in a world where urban areas are sprawling and open space for wind-energy harvesting—especially near energy-hungry population centers—can be hard to come by.

But there may be another way. Narrower, vertical-axis turbines, which resemble egg-beater blades more than propellers, eat up less space but convert less wind energy into the mechanical energy that produces electricity. Now Dabiri and his colleagues—inspired by how schools of fish swim—have a strategy that could significantly improve their output.

“This potentially could work in urban areas. But I think the bigger fish to catch is to use this for utility-scale power in the same way wind farms are designed,” says Dabiri, whose findings were published in the July issue of the Journal of Renewable and Sustainable Energy.

A 2010 MacArthur Fellow, Dabiri investigates fluid dynamics in air, water, even blood flow. Specifically, he observes the physics and mathematics on display in the vortices that movement generates in those media. And he has studied the way schools of fish align themselves in geometric patterns that decrease the chances that they will be in the path of another animal’s underwater wake.

Like a lot of researchers today, Dabiri is also interested in the development of energy sources to replace, or at least supplement, polluting fossil fuels. It occurred to him that vertical turbines might benefit from strategic placement of their own, but in this case in the path of their neighbors’ airflow wakes.

With Caltech graduate students Robert Whittlesey and Sebastian Liska, Dabiri first explored this idea with a mathematical model that yielded promising results, as reported last year in Bioinspiration & Biomimetics. More recently Dabiri supervised field testing of six 10-meter-tall vertical-axis wind turbines in California’s Antelope Valley.

Vertical turbines worked best when placed in stair-step-shaped patterns and when blades of alternating turbines rotated in different directions. That allowed them to extract energy from differently shaped vortices thrown by other turbines. Whereas wind farms stocked with horizontal-axis wind turbines produce two to three watts of power per square meter of land area, the field tests convinced Dabiri that vertical-axis turbines could produce 10 times as much in the same space.

Wind power is far from a major energy source in the United States. In 2010, it generated only 2 percent of the country’s electricity, according to the U.S. Department of Energy. Problems with mechanical reliability and energy storage persist, Dabiri says. Working with a start-up company in California to improve vertical turbines, he hopes to help change that.

Dabiri’s efforts fit into what researchers call bioinspiration, rather than biomimicry, says West Chester University marine biologist Frank Fish, who also looks to nature for useful innovation. Fish has examined the ways that tubercles, or bumps, on the flippers of humpback whales improve their control while swimming. His findings have inspired new takes on turbine blades, fans and surfboards.

Even if nature hasn’t solved the very same problem a researcher is tackling, Fish says, it can prompt ideas. “When looking at things like windmills, you might not get the same field of flow you find in fish. Still, you get the idea that fish align themselves,” Fish says. “You extrapolate.”